For many years the band saw blade industry has been using a steel alloy known in the trade as "D6A" as the backing steel in the manufacture of bimetallic band saw blades. The D6A alloy is an ultra high strength steel adapted to be used in the 260-290 ksi tensile strength range by hardening, or austenitizing, at 1550.degree. F. and tempering at 400.degree. F. These alloys were not effective for band saw blades when austenitized at 2000.degree.-2250.degree. F. and tempered at 800.degree.-1100.degree. F., the temperatures required for heat-treating the high speed cutting tool steel ("HSS") edge.
The problem of heat treating bimetallic blades comes about after the HSS alloy is welded generally by an electron beam welder along an edge of the backing band. Inasmuch as the HSS alloy of the cutting edge and the backing steel are welded together along the length of the blade, it is difficult, if not impossible, to obtain simultaneously the distinctive and necessary properties of HSS cutting tool steel on the one hand without bringing about a reduction of the flexibility, toughness and fatigue strength of the backing steel.. In recent years, while cutting hard-to-cut materials over protracted periods of time using bimetallic blades and when it has become necessary to replace the blade because of its failure, upon inspection of such blades, it has been determined that in many cases the failure occurred in the backing band and not the HSS cutting edge.
U.S. Pat. No. 2,565,264, which issued in 1951, relates to a tool steel with low alloy content, such as high speed steel 18-4-1. However, this patent does not relate to its use as the steel backing band for a bimetallic band saw blade, the subject of this invention, since that type of blade was not invented until about two decades later.
Among the more recent prior art: are U.S. Pat. Nos. 5,036,356 issued in 1991 to Kumagai and No. 5,091,264 issued in 1992 to Bohler which have directed their focus on alloys for use as the backing steel welded to high speed steel cutting edge material in fabricating bimetallic band saw blades. The patent to Bohler focuses on martensitically hardenable maraging steel containing, in relatively large quantities ranging from a minimum of 10% to a maximum 55% by weight total of the alloy elements, Ni, Co and Mo, which are three of the rarest and most expensive materials available. The Bohler patent discloses and claims that support strips 1 have at least 10% by weight of alloying components. In Table 1 (Column 4) and Table 3 (Column 50) the constituents of a tool steel and backing band, or support strip of Bohler are that generally the same materials are used in both the cutting and backing portions of such composite blades. The Bohler patent also discusses bimetallic band saw blades wherein a high speed steel HSS is electron beam welded to the backing strip. Usually, the welding step is followed by the tempering of that longitudinal section or zone hardened by the welding and by annealing steps. After the teeth are cut and set, the blade is hardened by heating to a temperature of 1120.degree. C. which is maintained for fifty-five (55) seconds and then quenching the blade in oil. The blade was then tempered by heating to 560.degree. C. and cooling in air for 1.2 hours and again by heating to 560.degree. C. and cooling in air for one hour. Blades of that construction provide 71,000 load cycles prior to failure, but include large amounts of the following three alloy elements, i.e., Mo - 4.3%; Ni - 18.1%, and Co - 12.07% by weight.
The '356 Patent discusses the problem of shorter blade life of such band saw blades because the blades are subjected to relatively large loads of tensile stress, bending and torsional stress particularly when used in cutting difficult-to-cut materials such as super heat resistant steel and stainless steel. That patent discloses as the backing steel of the invention, a material in which the carbon is combined with carbide forming elements such as Chromium (Cr), Molybdenum (Mo), Tungsten (W), Vanadium (V) and Niobium (Nb). Despite the fact that Chromium is a rare and expensive element, the patentee teaches that "Cr must be added in amounts in excess of 3% by weight for the purpose of increasing the hardenability of the backing material and preventing the diffusion of C (Carbon) from the backing material to the high speed steel during the welding of the backing material to the high speed steel . . . ." The '356 Patent also indicates that in the composition of the present invention, "if a Vanadium equivalent exceeds 0.2%, VC (vanadium carbide) and/or NbC (niobium carbide) excessively remain in the structure after heat treatment and there is a tendency to. . . and decrease the fatigue strength."
It is the principal object of this invention to provide an improved bimetallic welded edge band saw having a backing steel which possesses as good or better performance characteristics including fatigue life than any comparable product previously available or disclosed in the prior art.
It is another object of this invention to provide a bimetallic band saw blade of the above type which involves the use of substantially lesser amounts of certain rare and expensive alloying materials to thereby provide a lower cost high performance bimetallic band saw blade.
This invention comprises a bimetallic band saw blade having a HSS cutting edge welded to a backing band of steel alloy containing the following materials in percentages by weight as follows:
Cr - 0.5% to no more than 1.5% PA1 V - 0.10% to 0.75% PA1 Mo - 1.5% to 2.5%
the alloy also contains C, Si, Mn, Nb and/or Ni and Al in such varying amounts that the total alloy content, except for Fe and inevitable impurities, is in the range of 3.12% to 8.48%.
The above and other objects and advantages of this invention will be more readily apparent from the reading of the following description of an exemplary embodiment thereof taken in conjunction with the following drawings.